Alkoxycarbonyloxystyrenes, especially 4-t-butoxycarbonyloxystyrene, are valuable monomers used in the manufacture of specialty polymers. Incorporation of the monomer into a polymer backbone by polymerization via the olefin group affords a polymer with a pendant reactive functional group, i.e., the pendant alkoxycarbonyloxy group. An especially important area of use for 4-t-butoxycarbonyloxystyrene is in the manufacture of poly(4-t-butoxycarbonyloxystyrene) which is used in the manufacture of photoresists. Known methods for preparing alkoxycarbonyloxystyrenes employ expensive Wittig reaction chemistry. The process of the present invention affords an economical, one-pot synthesis route to these commercially valuable monomers.
Houlihan et al., Can. J. Chem., Vol. 63, pp. 153-162, (1985) discloses the t-butoxycarbonylation of phenols, alcohols, enols, thiols and phenolic polymers such as poly(p-hydroxystyrene) by reaction with di-tertbutyl dicarbonate under phase transfer conditions. Typical phase transfer catalysts such as the crown ethers can be employed, but the uncatalyzed reaction is preferred in the presence of potassium carbonate as it gives the t-butoxycarbonyloxy substituted polymer in quantitative yield while greatly simplifying purification. There is no mention of the t-butoxycarbonylation of monomeric styrenes or using substituted styrene compounds, such as acetoxystyrene, as a precursor.
U.S. Pat. No. 4,461,628 of Ito et al., issued Jan. 1, 1985 teaches the preparation of t-butoxycarbonyloxystyrene by reaction of p-hydroxybenzaldehyde with di-t-butyldicarbonate in the presence of base to yield p-t-butoxycarbonyloxybenzaldehyde, which is then reacted with methyltriphenylphosphonium bromide and potassium t-butoxide to yield the desired product.
Corson et al., J. Org. Chem., Vol. 23, pp. 544-549 (1958) disclose the preparation of vinylphenols and isopropenylphenols. In particular, the hydrolysis of p-acetoxystyrene to p-hydroxystyrene is taught to be quantitative under conditions so gentle that no purification of the p-hydroxystyrene is required.
Alkoxy styrenes have also been prepared by many other processes known in the art. Examples include palladium catalyzed cross-coupling, various equivalents of Wittig chemistry, dehydration of methyl phenyl carbinols, dehydrogenation of p-ethyl phenol and malonic acid synthesis.
The known methods for the preparation of alkoxycarbonyloxystyrenes are complex multi-step processes that commonly employ expensive reagents. They do not afford convenient one-pot synthetic routes to the alkoxycarbonyloxystyrene monomers. Some of these methods require the preparation of 4-hydroxy styrene as an isolatable intermediate. These methods suffer from the fact that 4-hydroxystyrene is known to be unstable, subject to ready and uncontrollable polymerization, and toxic.
Alternate routes to the commercially valuable polymers containing pendant alkoxycarbonyloxy groups depend on introduction of the alkoxycarbonyloxy group after formation of the styrene polymer. This approach necessitates isolation and purification of a polymeric product from the alkoxycarbonylation reaction mixture. Use of the polymers made from the monomers of the present invention in photoresists require a high degree of purity. It is well known in the art that it is preferable to prepare high purity polymers from high purity monomers rather than carry out chemical reactions and purifications with polymeric species.
It is therefore an object of the present invention to provide a process for the preparation of alkoxycarbonyloxystyrene from a readily available styrene.
It is a further object of the present invention to provide a simple one-pot, two-phase reaction for the alkoxycarbonylation of a substituted styrene with no isolation of an hydroxystyrene intermediate.
It is a further object of the present invention to provide an economical route for the preparation of t-butoxycarbonyloxystyrene.